Astronomers Find the Youngest Planet Ever Seen and Scientists Aren’t Sure How It Formed So Fast

Astronomers have identified a planet, Elias 2-24 b, that is the youngest known world, less than 1 million years old, and located 55 times farther from its star than Earth is from the Sun. The planet is situated in a narrow gap in a disk of gas and dust around its young star. The discovery challenges current theories about how giant planets form, as the planet's position and motion support the idea that it is part of the system. The study, published in The Astrophysical Journal Letters, found that the planet's motion and location inside the gap align with previous observations from the W. M. Keck Observatory. The planet's position and apparent motion strengthen the case that it belongs to the system, according to the study's lead author, Dr. Andrea Bernardi.

A Planet in a 30-AU-Wide Gap The gap helps explain what makes this discovery unusual. Elias 2-24 b orbits about 55 astronomical units from the star, or roughly 5.1 billion miles away, while the gap spans about 30 astronomical units. At that distance, building a giant planet by the usual process of assembling a solid core and drawing in gas is difficult to explain within the system’s short lifetime. Earlier observations with the Atacama Large Millimeter/submillimeter Array, or ALMA, mapped the gap in the disk. Other observations showed a faint source near it, raising the possibility of a planet. But a bright spot beside a young star can be difficult to identify. Dr. Bernardi and colleagues examined Keck observations from 2018 and reanalyzed another set from 2020. They recovered the source at both dates and found its position consistent with the earlier detections. They also tested whether it behaved like a fixed background object as the star moved across the sky. Their analysis found that the explanation was inconsistent with the measurements at a 3.2-sigma confidence level. Those images have limits. Neither Keck detection individually met the commonly used five-sigma threshold, and the researchers could not yet measure the planet’s orbital motion. Their confirmation rests on combined evidence: repeated detections, agreement with observations from other instruments, the source’s motion relative to the star, and its location inside the disk gap.

The gap alone never proved a planet. Elias 2-24 b gives astronomers a source they can place directly alongside the structure it appears to be shaping. Brightness Does Not Give a Simple Answer Researchers compared the object’s infrared brightness with models of young giant planets, estimating a mass of roughly 1.9 to 4 times Jupiter’s. Yet the paper cautions against treating that range as a precise measurement. A planet this young may still be pulling in material. Some of its light could come from that process rather than from heat retained inside the planet itself. Dust in the surrounding disk also affects what astronomers see. The researchers, therefore, describe their brightness-based mass assessments as upper limits. Models based on the gap’s shape allow a planet closer to Jupiter’s mass. This is one of the most revealing parts of the result. The bright point in the image is evidence of a forming world, but its glow does not tell astronomers its mass without assumptions about its atmosphere and how it is growing. Future observations at different wavelengths could help separate those effects. The system also puts the timing of planet development under pressure. Other directly imaged young planets cited in the study orbit stars around 5 million years old. Elias 2-24 is younger than 1 million years, while its planet orbits far beyond Jupiter’s distance from our Sun. “Our planet-formation models already struggled to explain the previous record holders for the youngest known planet,” co-author Dr. Lucas Cieza explained. “Elias 2-24 b shows us that even our best planet-formation models are still missing some important processes.” Researchers argue that the disk’s structure favors core accretion, in which smaller bodies grow into a

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